1. Field of the Invention
The present invention relates to a laser beam scanning apparatus and, more specifically, to a laser beam scanning aPParatus in which a laser beam from a laser beam generating source is deflected by a deflector, whereby a recording medium is scanned and an image is formed thereon.
2. Description of the Prior Art
Generally, optical apparatuses for printers employing a laser beam for carrying image information such as shown in Japanese Laid-Open Patent Application Nos. SHO 61-3114 and SHO 58-93026 are well known. In optical apparatuses of the above-mentioned type, a distance between a deflector and a recording medium is determined by the width of an image, said distance being over 200 mm for A4 size in general. An image reflecting mirror is used for making such a long optical path compact. The elements constituting the optical apparatus should preferably be provided as a unit, so as to facilitate the positioning of the apparatus to the main body of the printer, to reduce vibration and to facilitate the care of the apparatus such as exchange of parts.
On the other hand, compared with an electrophotographic copying machine (plane paper copier) utilizing projected images of originals by visible light, higher image quality is required for a laser beam printer. Therefore, the optical apparatus is a critical portion dominating the image quality, and various portions thereof should be adjusted when assembled.
Especially, the adjustments of the length of scanning line, which is a length of a laser beam track on the recording medium as well as of an illuminating position on the recording medium are important. Examples of methods for adjusting the scanning line length include one wherein the entire optical unit is moved relative to the recording medium, and one wherein the image reflecting mirror is moved in the direction of a light axis and the like. Compared with the former method, the latter one facilitates the adjustment as a optical unit and is preferable in view of the structure as a unit.
However, the movement of the image reflecting mirror for adjusting the scanning line length causes a problem in that the illuminating position on the recording medium is interlockingly altered. The change of the illuminating position caused by adjusting the scanning line length becomes a problem in the case of adjusting the scanning line length after the adjustment of the illuminating position or adjusting the scanning line length with the illuminating position fixed.